Before you can build an office building, a house, a mall, a parking lot, or any other structure, the site must be finished.
Estimating the amount of incision and filling required for evaluating a construction site is one of the most involved and tedious processes in building evaluation. The calculation of building materials, places for painting, the number of doors and windows, etc. Is a child's game compared to an estimate of the number of cuts and the number of fillings for the construction site. The reasons for this is that an unimproved site almost always has an irregular surface. Because of this, it is very difficult to determine the amount of dirt that must be removed from the unimproved area or entered into it.
There are three main ways to calculate the cutting and filling of the construction site. The first two are “at hand” and require a site plan, ruler, pencil, calculator and clock. The third method requires excavation software, a computer, and several computer skills.
In the first “on-hand” method, the grid method is used to calculate the cut and fill the area of the building. In this method, the site is divided into squares of a grid of equal size. The grid squares typically range from 1 to 2 inches on a paper plan. For each corner of each grid square, both the existing height and the estimated or final elevation height are calculated and indicated on the plan. Once all heights have been determined, the difference between the height and the proposed height in each corner is also indicated on the plan. Then, for each grid square, these differences in height for each of the four corners are added together and averaged by dividing the sum by 4. If the average value of the height difference for the grid square is negative, then this is the cut area. In the cut areas, existing heights should be lowered to the intended heights or “cut”. If the mean elevation difference is positive, it means that existing elevations must be raised to estimated heights or “filled”. Then all negative numbers are added together and summed to determine the total depth of the cut. Then the positive numbers are added together and summed as the total depth. Each of these totals and totals is then multiplied by the grid square area to determine the number of cubic feet of cut and cubic feet of filling. Finally, these cuts or fill volumes are divided into 27 to determine the total number of cubic meter sections of cubic holes and cubic filling yards (excavators operating in cubic yards, not cubic feet). Finally, the difference between the total volume of the cut and the total filling volume is calculated. If there is more incision than pouring, then the dirt should be removed from the construction site and taken to the truck. If there is more filling than cut, the dirt must be imported to the construction site. The time required for calculating the cut and filling with the grid method is about eight hours for one plan of a sheet section of average complexity. Accuracy with this method is +/- 20%.
The second method used to calculate the cut and fill is called the cross-section method. In this method, the appraiser draws a set on equally spaced horizontal lines according to the plot plan, divided from 1 to 2 inches. Then, for each cross-section line, the appraiser places on the graphic paper both the existing surface and the proposed surface relative to the distance along the cross-section. It then counts the number of grid squares where the existing surface is above the proposed surface. This is the incision area for this cross section. It then counts the number of grid squares where the existing surface is below the proposed surface. This is the filling area for the cross section. He repeats these steps until all cross-sections of the site plan have been completed. It then averages the area of the cut between each pair of cross sections and multiplies by the distance between the cross sections and dividends, the number of which is 27. This is the volume of the cut between adjacent cross sections. He then repeats these steps for the fill areas to determine the fill volume between all adjacent cross sections. He then summarizes all volumes of the section and all volumes of filling to determine the overall section and filling of the construction site. If there is more incision than pouring, the dirt should be removed from the construction site. If there is more filler than a cut, the dirt should be imported to the construction site. The time required to calculate the cut and fill with the cross-section method is approximately 20-30% more than the grid method. The accuracy of this method is about +/- 15%.
The third and final method of calculating the cutting and filling of the construction site is the use of excavation software. In this method, the height data is either tracked from the paper site plan, using a large-format digitizer, tracked on the screen, using a PDF plan of the building plan, or imported directly from an AutoCAD file. The computer then analyzes the drawing to build both existing and proposed surfaces. Then, for hundreds of thousands of points throughout the site, the software calculates the difference between the existing heights and the proposed heights to determine the total volume of the cut or fill for the construction site. The time required to calculate the incision and fill it with excavation software is about a quarter of the time required for the mesh method. Accuracy with this method is about +/- 5%.
The advantages of both the grid method and the cross section method for calculating the cut and filling are the use of inexpensive and widely available consumables. The disadvantage is that they require many hours of work and have moderate accuracy.
The advantage of using excavation software to calculate the cut and fill is much higher, estimating speed and much higher accuracy. The disadvantages are the cost, which can run from a few thousand dollars to a few tens of thousands of dollars for software and hardware and requires that the appraiser have some computer skills.
The appropriate method for your company to calculate cutting and filling out your applications depends on the number of departures you must complete each year. If you do only a few excavations a year, and you or your evaluator are not very busy, then the "on-hand" methods are probably adequate. If you are an excavator or general contractor who makes many estimates on cutting and filling, then the acquisition of the necessary equipment and software for excavation will greatly increase your ability and accuracy when bidding at workplaces that require excavation.
